Method of operating I.C. engines and apparatus thereof

ABSTRACT

Operation of unthrottled internal combustion engines is improved by providing the combustion chambers with an electrically heated glow plug having a catalyst surface layer on the ignition element. The catalyst is heated to and maintained at a temperature high enough to be effective for vaporization of fuel drops and ignition of vaporized fuel by controlled electrical heating. In operation of the engine air is compressed in a combustion chamber and at least a portion of the fuel is injected during the latter portion of the compression stroke and the injected fuel ignited by contact of fuel with the hot catalytic surface of the glow plug resulting in a combustion pressure wave in the immediate vicinity of top dead center.

This application is a Continuation-in-part of application Ser. No.311,848, filed Feb. 17, 1989, now U.S. Pat. No. 4,896,636.

BACKGROUND OF THE INVENTION

1. Field of the Invention

to an improved method of operating unthrottled internal combustionengines at compression ratios lower than required for diesel engines.Moreover, this invention relates to means for operating glow plugs inunthrottled engines at lower plug temperatures than would be requiredwith non-catalytic glow plugs of the same size and geometry. In onespecific embodiment, the plug temperature is provided with temperaturedetermining means and electrical power is controlled to maintain theplug walls at a value determined by the engine speed and power output.

This invention also relates to catalytic glow plugs capable of ignitingfuels at lower temperatures than a non-catalytic glow plug of the samesize and shape.

2. Brief Description of the Prior Art

Existing diesel engines achieve a significantly higher thermalefficiency than conventional gasoline engines in automotive use and emitacceptable levels of carbon monoxide and light hydrocarbons. However,soot and nitrogen oxide levels are high and compression ratios are muchhigher than the optimum for maximum fuel economy. Moreover diesels arerelatively hard to start as compared to automotive gasoline engines,even with electrically heated glow plugs, and require high cetane fuels.This is especially true of the lower compression diesels such as thelarge lower speed engines. With use of glow plugs, short plug life canbe a problem particularly under operating conditions which requirehigher plug operating temperatures, such as cold starting at arctictemperatures.

As a means of improving cold starting performance of conventional highcompression diesel engines with glow plugs, the use of catalyticallyself-heating glow plugs has been proposed (U.S. Pat. 4,345,555). Suchself-heating plugs are said to maintain a preset plug temperature byexothermic catalytic reactions after termination of the initialelectrical heating of the plug during starting. A self-heating plug issaid to maintain a higher temperature than a non-catalytic diesel glowplug and is further said to maintain a temperature above that requiredfor ignition of fuel by a non-catalytic plug. It is taught that thecatalyst should comprise a porous carrier, presumably to achieve greatersurface heating (it is well known that such porous supports provide agreater surface area for catalytic reactions than a non-porous support).Self-heating plugs can be expected to offer no improvement in plug lifeas compared to conventional glow plugs inasmuch as such self-heatingplugs are said to maintain a higher temperature than conventional plugs.Plugs which are effective at lower plug temperatures would allow easierstarting under adverseconditions and would enable starting lower ambienttemperatures.

In addition to the above cited shortcomings, conventional diesels cannotbe operated at low enough compression ratios for maximum efficiency andconventional diesels cannot efficiently utilize low cetane fuels such asmethanol and gasoline. Although in-cylinder catalysts previouslyproposed can improve efficiency and reduce emissions of soot andnitrogen oxides, retrofitting of existing engines is not alwayseconomically feasible, especially with small automotive diesels.

Conventional spark ignition engines are typically less efficient thandiesel engines in spite of operating in close appoximation to theconstant volume combustion Otto cycle, a more efficient cycle than thediesel cycle. This lower efficiency is believed to result primarily fromthe throttling losses associated with the requirement for sparkignition. Spark ignition requires near stoichiometric fuel-air mixturesfor flame propagation. To control power levels, the amounts of fuel andair must both be varied in step. This requires throttling of the inletair with resultant loss of pressure energy. Octane limits of fuelstypically limit compression ratios to below optimum levels. Operation ofspark engines without throttling of the inlet air could result in anengine more efficient than the diesel, even if such engines were limitedto below optimum compression ratios.

Attempts have been made to operate unthrottled engines at lower thandiesel compression ratios. With compression ratios too low forautoignition, an ignition source such as a spark plug or a continuouslyoperating glow plug is needed. Thus, stratified charge spark-ignitedengines of various designs, both piston and rotary, have been proposed.To date, such engines have not won acceptance. For use with heavy fuelssuch as diesel and jet A, spark plug fouling has been a severe problemleading to the use of glow plugs. Although use of glow plugs eliminatesthe fouling problem, a higher glow plug temperature is required foroperating a low compression ratio engine than for cold starting a dieselengine. This is believed to be because the compression temperature of alow compression engine is lower than that of a high compression dieselat typical cold start conditions. Another factor is that the ignitiontemperature of hydrocarbon fuels may be higher at lower pressures thanat higher pressures. With the high continuous operating temperaturerequired using conventional glow plugs in a low compression engine,typically in excess of about 1375 degrees K, plug heat losses must beminimal if plug power requirements are to be acceptable at all operatingconditions. With such a low heat loss plug it has been found that notonly is no electrical power required at full load operation but thatplug temperatures can even exceed the temperature limits of a hightemperature material such as silicon nitride. Although much larger plugscould be used to lower operating temperature to some extent, powerrequirements would be excessive and space might not be available. Thecapability to ignite fuels at lower compression temperatures hasimplications for cold starting of conventional diesels. Even withconventional high compression diesels, at low enough ambienttemperatures the compression temperature will be as low as in a 10/1compression ratio engine at the usually prevailing ambient temperatures.

The method of the present invention overcomes the limitations of theprior art by providing glow plugs capable of ignition at a surfacetemperature as much as 300 degrees Kelvin lower than required for anon-catalytic glow plug of the same size and configuration and byproviding an economical means of operating internal combustion enginesat lower compression ratios without throttling of the inlet air and thethrottling losses associated therewith. Use of the low ignitiontemperature catalytic glow plugs of the present invention in an internalcombustion engine enables quicker starts inasmuch as less time isrequired to heat a plug to a lower temperature. Equally important, byproviding a means of more rapid ignition at a lower plug temperature,combustion efficiency in engines is improved and emissions reduced. Itis believed that the lower ignition temperature and more efficientcombustion is a consequence of free radical production by the lowporosity catalytic ignition surfaces of the present invention. It isknown in the art that free radicals are combustion reactionintermediates.

SUMMARY OF THE INVENTION

According to the present invention, an internal combustion engine isfitted with a catalytic glow plug and control means to maintain thecatalytic surface of the glow plug at a specified temperature below thatrequired for rapid ignition of fuels with an equivalent geometrynon-catalytic conventional glow plug in the same engine. Typically thespecified temperature is 50 to 300 degrees Kelvin lower than for anequivalent non-catalytic plug, more preferrably 75 to 150 degrees Kelvinlower but may be as much as 600 degrees Kelvin lower with fuels whichare especially reactive catalytically. The catalytic surfaces of theglow plug may comprise a base metal oxide ignition catalyst or aplatinum metal catalyst. For best ignition performance, the catalystsurface is of low porosity, preferably substantially nonporous. Anessentially nonporous nature of the catalyst surface is advantageous toavoid permeation of fuel into the catalyst which would tend to cool thecatalyst on contact with injected fuel droplets. Advantageously, thecatalytic surface may be sintered at a temperature higher than theintended maximum operating temperature prior to use.

In operation of the engine, the glow plug is advantageously electricallyheated to bring it to the required operating temperature, typicallywithin the range of about 700 degrees Kelvin to about 1400 degreesKelvin depending on factors such as engine compression ratio, enginespeed, inlet air temperature and the fuel composition. Those skilled inthe art will appreciate that a specific optimal temperature foroperating a specific engine will be dependent upon the above factors,but can be readily determined by trial in the engine. After heating ofthe plug, the engine is started. Fuel is injected such that at least aportion of the fuel contacts the catalytic surface prior to the time ofmaximum compression. Electrical power is controlled such that the glowplug is maintained at a temperature appropriate for rapid ignition ofthe fuel at the given engine operating conditions. With lowercompression ratio engines, continued electrical heating is usuallyneeded at the lower power levels. Typically however, no electrical poweris required at full engine power operation because heat transfer fromthe hot combustion gases of fuel combusting at temperatures in excess of2000 degrees Kelvin is sufficient to heat even a non-catalytic plug to atemperature sufficient for ignition of fuel. Because catalytic oxidationof the fuel on the catalytic glow plug surface results in highconcentrations of ignition enhancing free radicals in the adjacent gas,a catalytic glow plug of the present invention requires a lower surfacetemperature for ignition of fuel and thus less electrical heating than anon-catalytic glow plug for rapid ignition of fuel. Operation of aninternal combustion engine in accordance with the present inventionoffers greater ease in starting and reduced emissions of soot, not onlywith conventional diesel fuels but also with low cetane fuels such asmethanol, ethanol, and other alcohols and oxygenated fuels. Coldstarting is made possible even at temperatures below 240K and even aslow as 210 or 200K provided the fuel is pumpable and the starting motorcan crank the engine. With a glow plug according to the presentinvention, oxygenated fuels such as methanol ignite even more readilythan diesel fuels. This is of considerable importance since it greatlyincreases the availability of fuels suitable for use in diesel enginesand in the more efficient lower compression ratio unthrottled internalcompression ratio engines made practical by use of glow plugs of thepresent invention.

In the present invention, improved ignition of the fuel by virtue ofcatalytic action is believed to result from surface oxidation of a minoramount of the fuel. It is believed that the catalyst injects radicalspecies into the gas phase, thus lowering the temperature required forgas phase combustion. It is well known that radical species can speed upcombustion. Accordingly, for effective ignition according to thisinvention, the required catalyst temperature is significantly lower thanrequired with a non-catalytic plug thus reducing the amount of electricpower required to achieve rapid ignition at low engine power levels in alow compression ratio engine. At full engine power output no electricalpower should normally be required. Even at compression ratios lower thanrequired for autoignition, at full engine power combustion temperatureshave been found to be high enough to maintain even non-catalytic glowplug surfaces at a temperature high enough for ignition of fuel with noelectrical power required.

At surface temperatures below those required for catalytic ignition ofthe fuel, the presence of a catalyst can even delay gas phasecombustion, thus the importance of controlling plug temperature. Thisresult is believed to stem from quenching of radicals generated in thegas phase. It has long been known that such quenching of free radicalsis promoted by active catalyst surfaces. It is believed that porouscatalytic surfaces are such poor ignition catalysts because the catalystpores can not only trap fuel but because the pores can trap freeradicals long enough for radical recombination, thus the need tominimize catalyst porosity. Conventional high surface area catalysts areparticularly ineffective for ignition, even though such catalysts aremuch more active than nonporous igniters for surface oxidation andsurface heating.

Although quenching of radicals has been suggested as a means to inhibitgum formation prior to spark or autoignition, the resulting inhibitionof combustion is disadvantageous inasmuch as such inhibition can quenchcombustion prior to completion resulting in high emissions ofhydrocarbons and carbon monoxide. In the present invention theelectrical power required may be reduced in low load operation by apilot injection of fuel immediately preceeding injection of the mainfuel charge or alternately by earlier timing of the injection of thefuel charge.

BRIEF DESCRIPTION OF THE DRAWINGS

This invention can be further understood with reference to the drawingsin which

FIG. 1 is a schematic of a system of the present invention.

FIG. 2 is a schematic of a predictive control system.

FIG. 3 is a sectional view of a conventional diesel glow plug which hasbeen modified by coating with an ignition catalyst.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS OF THE INVENTION

This invention relates to a method of operating a low compressionunthrottled engine wherein fuel and compressed air are contacted withthe catalytic ignition surface of a catalytic glow plug maintained byelectrical heating at a temperature sufficient for ignition of the fuel,whereby starting of the engine is facilitated and combustion efficiencyduring operation is improved even with engine compression ratios below14 to 1 or even with compression ratios below about ten or twelve toone. In one embodiment a catalytic coating is firmly affixed to thesurface of a conventional diesel glow plug. In another embodiment thewalls of the glow plug tip, ie: ignition element, are formed of acatalytic material, preferrably a catalytic base metal oxide ceramic. Instill another embodiment the glow plug is provided with temperaturedetermining means and the electrical power is controlled to maintain thewalls of the glow plug above a predetermined temperature. The catalysttypically comprises a base metal oxide or noble metal ignition catalyst.Injection of the fuel is timed such that at least a portion of the fuelcontacts the catalyst surface prior to the point of maximum compression.

More specifically, this invention relates to catalytic glow plugs andthe means to maintain a glow plug catalytic ignition element at anoperational temperature in an unthrottled low compression engine duringengine start-up and during operation at less than full load.

The present invention is further described in connection with thedrawings. As shown in FIG. 1, in one preferred embodiment the catalyticsystem consists of a glow plug 10 having an ignition element (tip) 11with a nonporous surface comprising an ignition catalyst and temperaturecontrol unit 12 which feeds power to plug 10 via line 13. Control unit12 determines the temperature of the catalytic surface of plug tip 11 bymeasuring the current and voltage applied to plug 10 and calculating theload resistance which is a function of the temperature of plug tip 11.Control unit 12 is designed to supply electrical power to plug 10 onlyas needed to maintain a predetermined temperature of tip 11, whichadvantageously may be a function of engine operating parametersincluding load, speed, and inlet air temperature. Advantageously, apreferred method is predictive control of the glow plug temperatureusing a computer. Control unit 12 may be a conventional unit known inthe art, as for example such as the Condarcure units availablecommercially. As shown in FIG. 2, process control computer 22,hereinafter referred to as the predictive controller, is preprogrammedto supply power to glow plug 10 during starting and thereafter to supplypower as a function of the power level setting of engine fuel injectorpump 23, typically maintaining plug tip 11 at a temperature at leastabout 75K lower than required at lower power settings of fuel injectorpump 23 than would be required for a non-catalytic glow plug. Controller22 may be connected via line 26 to optional inlet air temperature sensor27 and programmed to apply increased electrical power at lower inlet airtemperatures. During engine operation controller 22 monitors plugcatalyst temperature by measurement of the current and voltage appliedto plug 10 as described above. The computer 22 is a conventionalhardware item commercially available.

FIG. 3 shows an expanded sectional-in-part view of a conventional glowplug 30 to which a coating of a refractory metal oxide 32 has beenapplied by sputtering to plug tip 31. The metal oxide preferrably has amelting point of at least about 2000K. To maximize thermal shocktolerance and to minimize thermal lag it is preferred that coating 32 bethin, less than 10 mils thick and preferably less than 2 mils or evenless than 0.5 mils. Only a minimal thickness is required, as for example0.0001 mils. Ignition catalyst 33 comprises an overcoating of a portionof the surface of coating 32; preferably a major proportion (at least 51percent). Suitable ignition catalysts include the low vapor pressureplatinum group metals, such as Pt, Pd, Rh and the like; refractory basemetal oxide ignition catalysts, such as CoO, NiO, and the like and hightemperature stable base metal oxide compounds such as the perovskites.Alternatively, the oxide coating 32 may itself comprise the catalyticsurface 33 if a catalytic material is used for coating 32. Methods ofapplying suitable catalytic coatings are known in the art. Especiallyadvantageous for the purposes of this invention is the ignition catalystcoating and method described in U.S. Pat. No. 4,603,547, incorporatedherein by reference thereto.

For enhancement of diesel engine ignition, it is important that thecatalytic glow plug be maintained at a temperature at which the catalystused is effective for ignition of the fuel. In general the plug tip ismaintained at a temperature of about 75 to 300 degrees Kelvin lower thanthat required to start a diesel engine using a conventional glow plug.The required plug temperature is readily determined for any fuel bycontacting a flammable fuel air mixture with a heated glow plug. Thecontrol means is then readily designed to maintain the catalytic glowplug at a temperature at which the catalyst is effective for rapidignition. Preferrably, during cold starting of an engine the glow plugis maintained at a temperature at least about 50 degrees Kelvin higherthan the desired control temperature during normal operation of theengine. This allows faster start-ups. During normal operation afterengine start-up, electrical power need be supplied to the catalytic glowplugs only if the plug temperature falls below the predetermined controltemperature. It should be understood that during full load operation ofthe engine, combustion of fuel in the engine will typically maintain thecatalytic plug at a temperature above the control temperature withoutthe necessity of electrical heating. With conventional diesels notrequiring glow plugs once started, use of the catalytic plugs of thisinvention as a starting aid, allows cold starting of a cranking engineat ambient temperatures as low as 200K. Even with engine compressionratios below about 12 to 1, it may be possible to operate at idlewithout electrical heating after initial start-up, especially with anoxygenated fuel as for example methanol.

With conventional glow plugs, the high temperatures required foreffective surface ignition of fuels not only impose a high powerrequirement but shorten glow plug life if the plugs are kept incontinuous operation. Use of catalytic glow plugs in accordance with thepresent invention reduces power requirements in continuous operation byreducing the surface temperature required for ignition of fuels. Glowplugs with oxide ceramic ignition elements are advantageous because suchplugs can better tolerate the very high temperature oxidizing conditionstypical of full load operation. It is feasible to better insulate such aplug against heat loss, further minimizing the electrical power requiredat low load. With plugs of conventional materials it has been foundnecessary to allow sufficient heat loss to limit full load maximumtemperature to an acceptable level, ie: about 1500 degrees Kelvin orless. Moreover, such conventional plugs having a relatively narrowoperating temperature range impose severe design requirements on thetemperature controller to avoid overheating of the plug shouldelectrical heating requirement change abruptly as during a rapid engineacceleration. Glow plugs capable of igniting fuels at lower temperaturesgreatly simplify controller design and make it possible to assure thatthe plug temperature will not exceed allowable limits. Cooling fins onthe plug body may be employed to limit maximum plug temperature by heattransfer to the ambient air.

The following examples describe the means of making and using theinvention and set forth the best mode contemplated for carrying out theinvention but are not to be construed as limiting.

EXAMPLE 1: To demonstrate the superiority of low ignition temperaturecatalytic glow plugs under adverse operating conditions, an NGK dieselglow plug was obtained and a thin non-porous coating of alumina appliedby sputtering to the ignition surfaces. An aqueous solution containingchloroplatinic acid was then applied to the alumina surface and the plugheated electrically to activate the platinum. The coated plug was thencompared to an uncoated NGK plug in a John Deere rotary engine inDeere's 20-1 test cell. Operating at 4800 RPM and 17 percent of fullload power output, with the catalyst coated plug in the test rotorchamber the engine operated satisfactorily at plug temperatures as lowas 1045 degrees Kelvin. With the conventional uncoated NGK glow plug inthe test rotor chamber, the engine could not be operated as low as 17percent power output even with the plug temperature at 1336 degreesKelvin. With the engine operating at a higher power level with theuncoated plug at 1336 degrees Kelvin, reduction in the power settingresulted in combustion failure and engine shutdown.

EXAMPLE 2: To test the durability of catalyst coated conventional glowplugs, four Volkswagen (VW) diesel glow plugs were coated with a thincoating of zirconia (less than about 2 mils thick) and a major portionof the surface was then additionally solution coated with aplatinum/alumina/zirconia catalyst composition. One of the glow plugs ofa diesel Rabbit engine was then replaced with one of the catalyticplugs. After a several thousand miles the catlytic plug was removed forexamination. The plug removed showed no visible signs of damage.

EXAMPLE 3: In accordance with the present invention, a four cylinderdiesel engine of a VW Rabbit is modified by replacing the conventionalglow plugs in the combustion chamber of each cylinder with catalyticglow plugs as used in the durability test just described in Example 2,and by changing the head gasket to lower the compression ratio belowabout 14/1. In operation of the engine the ignition catalyst surface ofthe plug is electrically heated to a temperature high enough to beeffective for vaporization of diesel fuel and ignition of vaporizedfuel. The electrical heating is applied as needed using a predictivecontroller to maintain the catalyst at a predetermined operatingtemperature at least about 150 degrees Kelvin lower than required forignition of diesel fuel using a non-catalytic glow plug in the engine.Air is compressed in the combustion chamber and diesel fuel is injectedin the normal manner at a time approximately three crank angle degreeslater than recommended by VW. The fuel is ignited by contact with theheated catalyst with the resulting combustion resulting in a combustionwave in the vicinity of top dead center with minimal formation of soot.

What is claimed is:
 1. A method for more rapid starting of anunthrottled internal combustion engine, which method comprises:(a)electrically heating an ignition catalyst comprising at least aneffective portion of the ignition surface of a glow plug to atemperature effective for vaporization and ignition said temperaturebeing at least about 75 degrees Kelvin below that value required forignition with a non-catalytic glow plug of the same size andconfiguration as said catalytic plug; and (b) igniting gas phasecombustion of an admixture of fuel and air by contact of said admixturewith said heated catalyst, said combustion resulting in a combustionwave in the immediate vicinity of top dead center.
 2. A method forigniting fuel, which method comprises:(a) electrically heating anignition catalyst comprising at least an effective portion of theignition surface of a glow plug to a temperature effective forvaporization and ignition of said fuel, said temperature being at leastabout 75 degrees Kelvin below that value required for ignition with anon-catalytic glow plug of the same size and configuration as saidcatalytic surface plug; and (b) igniting gas phase combustion of anadmixture of said fuel and air by contact of said admixture with saidheated catalyst.